Consider for example module subplugins such as a Database module field. This subplugin has no designated plugin type in pluginlib and hence falls under the general type plugintype_general. But plugintype_general is empty and the parent plugintype_base offers only minimal means of plugin management. In particular it doesn't offer plugin settings.
Is there a good reason for that, or is there API underway?
After all, there is no limit to the possible plugin and subplugin types in moodle and there should be some means for managing them (uninstall, settings, etc.).